There has been a proliferation of devices requiring power sources that can be charged and discharged, such as hybrid electric vehicles, which operate on both energies of gasoline and electricity, uninterruptible power supplies, mobile communications devices, and portable electronic devices. This has lead to a significant increase in the demand for high performance power storage devices for use as the power source for such devices. Specifically, there is a demand for high performance power storage devices having a high output, a high capacity, and good cycle characteristics.
Various attempts have been made to heighten the performance of power storage devices. In particular, since an increase in the energy density of electrode active materials leads directly to an increase in the energy density of power storage devices themselves, electrode active materials are extensively studied. For example, organic compounds having a conjugated π electron cloud have been proposed as new electrode active materials that are expected to be charged and discharged at high rates (see PTLs 1 to 6).
PTL 1 discloses organic compounds with a conjugated π electron cloud, such as tetrathiafulvalene represented by the chemical structural formula (Q1) (hereinafter referred to as “TTF”). TTF has a high energy density of approximately 260 mAh/g.

PTL 2 discloses organic compounds with a conjugated π electron cloud, such as 2,2′-ethanediylidene-bis-1,3-dithiol represented by the chemical structural formula (Q2) (hereinafter referred to as “EBDT”). EBDT has a high energy density of approximately 230 mAh/g. Also, the speed of the electrochemical reaction of EBDT is very high.

An electrolyte often contains a non-aqueous solvent (organic solvent) having the property of dissolving an organic compound of a low molecular weight. Therefore, in a system using an organic compound as an electrode active material, it may become necessary to suppress the dissolution of the electrode active material into an electrolyte. To solve this problem, PTL 3 discloses an electrode active material in which an organic compound having a conjugated π electron cloud is bonded to the main chain of a polymer compound. Examples of the polymer compound include polyacetylene and polymethyl methacrylate. An example of the organic compound having a conjugated π electron cloud is TTF.
PTL 4 discloses a TTF trimer represented by the chemical structural formula (Q3). Since the TTF trimer has an extended planar structure comprising a plurality of TTF skeletons, it is difficult to dissolve in an electrolyte. Therefore, an electrode active material for a power storage device including a TTF trimer has a high capacity, a high output, and good cycle characteristics.

PTL 5 discloses organic compounds having skeletons represented by the chemical structural formulae (Q1), (Q2), and (Q3). Since these organic compounds have a planar molecular structure and have a conjugated π electron cloud in the direction perpendicular to the planar molecular structure, they tend to become crystalline upon charge/discharge. The use of an electrode active material including such an organic compound can provide a power storage device having a high capacity, a high output, and good cycle characteristics.
PTL 6 discloses a polymer having a main chain that includes the TTF skeleton represented by the chemical structural formula (Q1) as a repeat unit, and this polymer does not dissolve in an electrolyte. The use of the polymer of PTL 6 as an electrode active material can provide a power storage device having a high capacity, a high output, and good cycle characteristics.